We study the physics of hard-core bosons with unfrustrated hopping (t) and nearest-neighbor repulsion (V) on the three dimensional pyrochlore lattice. At half-filling, we demonstrate that the small V/t superfluid state eventually becomes unstable at large enough V/t to an unusual insulating state which displays no broken lattice translation symmetry. Equal time and static density correlators in this insulator are well described by a mapping to electric field correlators in the Coulomb phase of a U(1) lattice gauge theory, allowing us to identify this insulator with a U(1) fractionalized Mott-insulating state. The possibility of observing this phase in suitably designed atom-trap experiments with ultracold atoms is also discussed, as are specific experimental signatures.

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ABSTRACT. Modelling the response of Himalayan glaciers to rapid climate change is an important problem. The poorly understood effects of debris cover and the variable response of the glaciers have made it difficult to understand their dynamics. We propose a simple model for debris-covered glaciers and validate it against data from Dokriani Glacier, India. Numerical investigations of the model show that the response of debris-covered glaciers to a warming climate has two timescales. There is a period when the glacier loses ice by thinning but the front is almost stationary and it develops a long, slowflowing tongue. This stationary period, which can be >100 years for glaciers with a large extent of debris cover, is negligible for bare glaciers. The quasi-stagnant tongue does not develop in response to cooling. An analysis of remote-sensing data in the light of these results indicates that the variable response of the glaciers in the Himalaya is consistent with a climate that is warming on average, but has considerable spatial variability in the warming rates. We estimate the average warming rate to be about the same as the global average.

The spin texture surrounding a non-magnetic impurity in a quantum antiferromagnet is a sensitive probe of the novel physics of a class of quantum phase transitions between a Neel ordered phase and a valence bond solid phase in square lattice S=1/2 antiferromagnets. Using a newly developed T=0 Quantum Monte Carlo technique, we compute this spin texture at these transitions and find that it does not obey the universal scaling form expected at a scale invariant quantum critical point. We also identify the precise logarithmic form of these scaling violations. Our results are expected to yield important clues regarding the probable theory of these unconventional transitions.Comment: 4+ pages, 2 columns, 4 figure

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